This invention relates generally to an optical recording system and more particularly to an optical recording system employing a solid immersion lens having an aspherical surface that tends to increase the optical system tolerance.
Optical data storage systems are of great commercial and academic interest due to their potential for very high data density storage. Unlike magnetic recording, where data density may be limited by particle size, the density of optical recording is limited by the size of the laser beam spot. Any technological improvement that tends to decreases the size of the laser beam spot may be used in a optical recording system to increase the data density.
According to the diffraction theory, when a laser is focused by a lens, the minimum laser beam spot size is the diffraction limit of the lens. The diffraction limit of the lens is proportional to the laser wavelength and inversely proportional to the numerical aperture (NA) of the lens. Decreasing the wavelength of the laser theoretically will decrease the diffraction limit and thus tend to result in a smaller laser beam spot size.
Another prior art method of reducing the laser spot size is to increase the NA of the lens. As will be appreciated, solid immersion lenses (SILs) can possess a very high NA. An optical system employing SIL could greatly increase NA and therefore greatly increase optical recording density. An optical system employing SIL also includes an objective lens for focusing the laser beam on the SIL.
SILs have a curved surface and a flat surface. The curved surface of the SILs are typically partial-sphere shapes such as a partial hemisphere less than half a sphere; or, a hemisphere exactly one half of a sphere; or, a truncated sphere more than a hemisphere, also known as a super-hemisphere. U.S. Pat. No. 5,881,042, by Knight (Knight) is one example utilizing spherical SILs. Knight is hereby incorporated by reference herein for all purposes.
In an SIL with a spherical curved surface, the assembly tolerance is severely limited due to rapidly increasing aberration when the lens position deviates from ideal position, especially in the optical axis direction. To control these tolerances, the objective lens and the SIL are rigidly mounted in an assembly which assures the distance between the objective lens and the spherical SIL remains constant. This results in a reading and recording head that is difficult and expensive to manufacture and heavier than ideal.
The present invention provides an improved optical reading and recording system employing an aspherical solid immersion lens (SIL). The improved optical system has an aspherical SIL between an objective lens and the recording media. The aspherical SIL reduces the focused electromagnetic radiation beam spot size by providing an increased numeric aperture (NA) of the optical system. The aspherical surface of the SIL reduces aberrations thereby providing greater manufacturing and operating tolerances. Greater manufacturing tolerances provide an easier to manufacture optical system. Increased ease of manufacture reduces the cost of optical systems.
According to the present invention, the objective lens and the aspherical SIL are no longer required to be rigidly mounted in one assembly as in the prior art. Utilizing an aspherical SIL allows the objective lens to be mounted in the arm and the aspherical SIL to be mounted in a separate, lighter weight, air bearing slider operating very near the recording media. A lighter weight, air bearing slider requires a reduced air bearing to operate.